There are currently in the market a number of zirconium oxide depositing conversion coating products useful as coatings in, for example automotive and appliance assembly lines. One use is to replace zinc phosphate products, which are coining under increasing environmental regulation. These zirconia depositing products are used to coat ferrous metal surfaces as well as aluminum and zinc containing surfaces.
Typical zirconium oxide conversion coatings deposited on metal substrates result in a zirconium deposition of 1 to 50 mg Zr/m2, with commercial benchmarks being in the range of 20 to 45 mg Zr/m2. This is a low coating weight as compared to zinc phosphate conversion coating which is understood in the art to generally provide coating weights in the range of 2 to 5 grams/m2 measured as total coating weight, which is generally considered to be roughly half phosphorus and oxygen, the remainder being zinc and various optional transition metals depending on the zinc phosphating product, thus providing a coating weight of about 1000-2500 mg/m2 measured as Zn. One drawback of zirconium oxide coatings at zirconium deposition of 1 to 50 mg/m2 is that they do not provide as good corrosion protection as conventional zinc phosphate processes under some circumstances. Thus there is a need for compositions and processes of coating substrates with a thicker layer of zirconium oxide conversion coating that provides improved corrosion performance
Another prospective use for zirconium oxide depositing conversion coating products is as replacements for iron phosphate depositing “cleaner/coater” products. These known phosphate-based cleaning products etch ferrous metal surfaces and produce an iron phosphate coating in addition to cleaning contaminants from surfaces. Generally “cleaner/coater” products provide a coating only on the ferrous metal surfaces and clean but do not coat non-ferrous surfaces, such as surfaces of zinc, zinc alloy, aluminum and aluminum alloy. Also, to produce these iron phosphate coatings, the “cleaner/coater” must include a source of phosphate, which is considered a drawback due to environmental concerns regarding phosphate discharge which is regulated and requires more costly waste treatment processes. Thus there is a need for replacement of iron phosphate cleaner/coaters (clean and pretreat the substrate from the same bath) with more environmentally-friendly, low phosphate or phosphate-free, low temperature, cleaner/coaters. There is also a need for a composition that can provide a conversion coating and cleaning of multi-metal substrates, such as ferrous metal and one or more of zinc, zinc alloy, aluminum and aluminum alloy.
Attempts have been made in the past to prepare compositions that clean a substrate and deposit zirconium oxide containing conversion coatings from the same bath but Applicants have found that many surfactants added to commercially available zirconium oxide depositing pretreatment compositions resulted in unfavorable lowering of coating weights and/or poor corrosion performance of coated substrates, as compared to the surfactant free commercial bath.
Some surfactants added to commercially available zirconiumn oxide depositing pretreatment concentrate and/or bath compositions resulted in instability of the compositions including precipitation of solids and/or separation into two or more liquid phases.
The instant invention solves one or more of the above problems in zirconium oxide coating compositions and processes by including in the pretreatment bath a combination of surfactants that allows depositing an adherent zirconium oxide conversion coating on a substrate in the absence of prior cleaning of the substrate and/or deposits a zirconium oxide conversion coating on multi-metal substrates at a higher coating weight for a selected contact time, as compared to a substrate conventionally cleaned in a separate step and contacted with a conventional zirconium oxide producing coating bath for the same contact time.